This invention relates to machine tools and more particularly to adjustable lapping tools for lapping the inside diameters of small holes to extremely close tolerances.
In the continuing effort to increase the life of rotating parts which are subject to wear, there is a need to be able to quickly size and finish holes by lapping them to a greater accuracy than heretofore provided. For example, it is now required that holes for valves and other hydraulic parts in the aircraft and space industries be lapped to an accuracy of as great as five to ten millionths of an inch so as to create a maximum percentage of bearing area. Thus, as the tolerances on the job become closer, it becomes more essential that the tooling used for internal lapping comes closer to meeting the ideal requirement of the bore.
Previously in the art, internal lapping tools have been made by providing a cast iron sleeve with a tapered hole therethrough and a helical slot therealong. The tapered hole enables the sleeve to be mounted on a mated tapered mandrel. The advancement of the sleeve on the tapered mandrel to enlarge its diameter during the coarse of the lapping operation is accomplished by manually striking the end of a lap expander positioned adjacent the sleeve or by manually operating a screw expansion device. Although this appears to provide a uniform enlargement of the diameter throughout the cylindrical lapping sleeve as needed for accurate lapping, actually such a construction and structural arrangement is ineffective when it is desired to accurately size the internal wall of a hole to a tolerance of a few millionths of an inch. This is because of minute variations of expansion that such uncoiling produces along the length of the sleeve. Furthermore, such a lapping tool has the disadvantage that the cylindrical lapping surface of the sleeve is not continually kept snugly up against the wall of the hole as the sleeve wears and the hole enlarges but must be incrementally adjusted every so often by manually striking the end of the lap expander, for example.